Apparatus and method for producing float glass having reduced defect density

ABSTRACT

A float glass chamber and related methods comprising a hot section having an atmosphere in at least the lower plenum comprises less than 3 percent hydrogen based on volume and a cold section, wherein the boundary line between the hot section and the cold section is where the temperature of the glass falls below a threshold temperature

RELATED APPLICATION

[0001] This application claims the benefits of U.S. ProvisionalApplication Serial No. 60/414,516 filed Sep. 27, 2002, which applicationis herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a float glass chamber used toproduce flat glass by the float glass process, and more specificallyfloat glass chambers that can be used to yield glass having reduceddefect density.

BACKGROUND

[0003] The float glass process is well known for making sheets of glass.In a typical float glass process, batch materials are heated to formmolten glass. The molten glass is then poured onto a bath of molten tin.The molten glass is drawn along the bath of molten tin andsimultaneously cooled and attenuated to form a dimensionally stablecontinuous sheet of glass, typically referred to as a glass ribbon. Thesheet is then removed from the bath for further processing.

[0004] Two types of furnaces are used in the float glass process—anair-fuel furnace and an oxy-fuel furnace. In an air-fuel furnace, fuelis mixed with warm air and combusted to provide heat to melt the glassbatch materials.

[0005] In an oxy-fuel furnace, oxygen, not air, supports combustion. Asa result, an oxy-fuel furnace provides a much more efficient melt thanan air-fuel furnace because energy is no longer being wasted heated upnitrogen in the air and oxy-fuel flames have a higher flame temperaturewhich radiates more efficiently. The increased melting efficiency allowsmore tonnage to be processed through an oxy-fuel furnace than through asimilarly sized, air-fuel furnace.

[0006] Both air-fuel and oxy-fuel furnaces have water in theiratmospheres. The head space (the area of the furnace above the moltenglass) in an oxy-fuel furnace has a higher concentration of water thanin an air-fuel furnace because the oxy-fuel atmosphere lacks thenitrogen provided in an air-fired furnace that dilutes the total waterformed by combustion. Stoichiometrically, the water typicallyconstitutes about 66% by volume of the head space in an oxy-fuel furnaceversus 18% in an air-fired furnace. Since the amount of water in theglass melt is proportional to the square root of the concentration ofwater in the head space, glass melted in an oxy-fuel furnace has a 1.7to 2 times higher water concentration than glass melted in aconventional air-fuel furnace. Typically, glass melted in an oxy-fuelfurnace contains more than 0.045 weight percent water based on the totalweight of the composition.

[0007] At the stage of the float glass process where molten glass ispoured onto molten tin, the molten tin temperature in the float bathranges from 1800° F. to 1900° F. (981° C. to 1037° C.). At 1800° F., atthe glass-tin interface, water that diffuses out of the molten glassdissociates into hydrogen and oxygen. Because hydrogen isn't verysoluble in tin at 1800° F., much of the hydrogen does not dissolve inthe tin but remains in the atmosphere of the bath. Some of the hydrogenfrom the disassociation of water gets trapped at the interface betweenthe molten glass and tin and ultimately impinges on the bottom surfaceof the glass ribbon and form defects along the ribbon surface typicallyreferred to as open bottom bubbles. The open-bottom bubbles can bedescribed as voids in glass that generally have an inverted-U shapecross-section. The presence of open bottom bubbles increases the overalldefect density of the glass.

[0008] Customers set requirements for the defect density of glass forcertain applications. The standards are very difficult to meet withconventional float glass processes due to the presence of open bottombubbles.

[0009] The present invention provides a novel apparatus and method thatyields float glass having a lower total defect density as a result ofreduced open bottom bubble defects.

SUMMARY OF THE INVENTION

[0010] In one embodiment, the present invention is float glass chambercomprising:

[0011] a hot section having an atmosphere in at least the lower plenumcomprises less than 3 percent hydrogen based on volume; and

[0012] a cold section, wherein the boundary line between the hot sectionand the cold section is where the temperature of the glass falls below athreshold temperature.

[0013] In another embodiment, the present invention is method for makingfloat glass with reduced defect density comprising:

[0014] a. melting a glass composition to form a glass melt; and

[0015] b. pouring the glass melt in a float chamber having a hot sectionand an cold section, the boundary line between the hot section and thecold section is where the temperature of the glass falls below athreshold temperature,

[0016] wherein the hot section has an atmosphere in at least the lowerplenum comprises less than 3 percent hydrogen based on volume

DRAWINGS

[0017]FIG. 1. is a sectional view of a float chamber according to thepresent invention, with portions removed for clarity.

DESCRIPTION OF THE INVENTION

[0018] As used herein, spatial or directional terms, such as “left”,“right”, “inner”, “outer”, “above”, “below”, “top”, “bottom”, and thelike, relate to the invention as it is shown in the drawing figures.However, it is to be understood that the invention may assume variousalternative orientations and, accordingly, such terms are not to beconsidered as limiting.

[0019] Unless otherwise indicated, all numbers expressing dimensions,physical characteristics, quantities of ingredients, reaction conditionsand so forth, used in the specification and claims are to be understoodas being modified in all instances by the term “about”. Accordingly,unless indicated to the contrary, the numerical values set forth in thefollowing specification and claims can vary depending upon the desiredproperties sought to be obtained by the present invention. At the veryleast, and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Moreover, allranges disclosed herein are to be understood to encompass any and allsubranges subsumed therein. For example, a stated range of “1 to 10”should be considered to include any and all subranges between (andinclusive of) the minimum value of 1 and the maximum value of 10; thatis, all subranges beginning with a minimum value of 1 or more and endingwith a maximum value of 10 or less, e.g., 5.5 to 10 or 3.2 to 7.8.

[0020] Conventional float glass processes are typically carried outusing a float chamber as shown in FIG. 1. Non-limiting examples of floatglass processes are disclosed in U.S. Pat. No. 3,083,551, U.S. Pat. No.3,961,930, and U.S. Pat. No. 4,091,156, which are all herebyincorporated by reference.

[0021] In a conventional float glass process, a glass batch compositionis heated to a molten state and poured into the float chamber.Typically, the float chamber has a refractory roof 3 that divides thechamber into an upper plenum 1 and a lower plenum 2. The lower plenumcontains the glass 4 and the tin 5. The upper plenum contains all of theoverhead electrical heating elements to provide controlled heating ofthe liquid metal float bath and the formed glass ribbon. A controlledatmosphere is maintained in the chamber via gas inlets 6 and gasoutlet(s) 7.

[0022] The novel float glass chamber of the present invention comprisesat least two sections—a hot section and a cold section. The boundaryline between the hot section and the cold section is where thetemperature of the glass falls below a predetermined temperature,hereinafter referred to as the “threshold temperature,” required forglass in the hot section. In a non-limiting embodiment of the presentinvention, there is no physical barrier between the hot section and thecold section.

[0023] In one non-limiting embodiment of the invention, the thresholdtemperature is 2100° F. In another non-limiting embodiment of theinvention, the threshold temperature is 1800° F. In another non-limitingembodiment of the invention, the threshold temperature is 1600° F. Thelower the threshold temperature for the hot section, the larger the hotsection and the smaller the cold section and visa versa.

[0024] In a non-limiting embodiment of the present invention, the hotsection of the chamber is approximately 90 to 100 feet from the pointwhere the molten glass is poured onto the tin. The cold section of thechamber is the next approximately 70 to 140 feet of chamber behind thehot section, depending on the size of the bath.

[0025] In a non-limiting embodiment of the present invention, numerousgas inlets and outlets are present in the upper plenum and lower plenumof the float chamber. Various gaseous mixtures can be pumped into thechamber through the gas inlets or out of the chamber through the gasoutlets to control the atmosphere within the chamber.

[0026] In a non-limiting embodiment of the invention, the gas inlets toat least the lower plenum over the hot section of the chamber deliver ina gas comprising less than 1 weight percent hydrogen based on volume.The remainder of the gas can be an inert gas, such as but not limited tonitrogen. Under normal operating conditions, in one non-limitingembodiment of the present invention, the atmosphere of the lower plenumover the hot section of the chamber can comprise 3 percent hydrogenbased on volume. In another non-limiting embodiment of the presentinvention, the atmosphere of the lower plenum over the hot section ofthe chamber can comprise 1 percent hydrogen based on volume.

[0027] Various mixtures of hydrogen and nitrogen or argon or ammonia inplaced of mixed gases can be pumped into the atmosphere of at least thelower plenum over the cold section of the chamber. In a non-limitingembodiment of the invention, the gaseous mixture can comprise up to 10percent of the hydrogen based on volume. The rest of the gas can benitrogen.

[0028] The gas outlets in the float chamber can be used to remove gasfrom the chamber. In one non-limiting embodiment of the invention, up to40 volume percent based on volume of the total flow of the gas pumpedinto the chamber as discussed above can be removed from the hot section.In this embodiment, it may be necessary to adjust the level of nitrogenin the atmosphere to prevent hydrogen from flowing upstream into the hotsection of the chamber.

[0029] By reducing the hydrogen in the hot section of the float chamber,the present invention reduces the level of saturation of molten tin,specifically with respect to hydrogen, at the hot section of the floatchamber. The molten tin is able to dissolve more hydrogen from thedisassociation of water so open-bottom bubble defects in the glass arereduced.

[0030] The present invention also encompasses a method for producingglass. According to the present invention, glass can be produced via thefollowing steps: adding glass batch materials to a furnace; melting thebatch materials; pouring molten glass from the furnace into the floatchamber; and removing the float glass from the float chamber.

[0031] The first step of the present invention comprises adding glassbatch materials to a furnace. The furnace can be an air-fuel furnace oran oxy-fuel furnace. The glass batch materials can be of anyconventional type including, but not limited to, conventionalsoda-lime-silica glass batch materials. A conventional glass compositioncan be characterized as follows:

[0032] from 65 to 75 weight percent SiO₂;

[0033] from 10 to 20 weight percent Na₂O;

[0034] from 5 to 15 weight percent CaO;

[0035] from 0 to 5 weight percent MgO;

[0036] from 0 to 5 weight percent Al₂O₃;

[0037] from 0 to 5 weight percent K₂O; and

[0038] from 0 to 2 weight percent Fe₂O₃.

[0039] All values are in weight percent based on the total weight of theglass composition.

[0040] The second step of the present invention comprises melting thebatch materials in the furnace. The melting processes can beaccomplished using techniques that are well known in the art. Forexample, in an oxy-fuel furnace, the batch materials can be melted bysupplying oxygen and fuel to melt the batch materials.

[0041] The third step of the present invention involves pouring moltenglass from the furnace into the float chamber. As is well known in theart, the molten glass flows onto the top of the molten tin and movesalong the top of the tin from the hot section of the chamber to the coldsection of the chamber. The temperature of the glass in the hot sectionand the cold section of the chamber are as discussed above. Also, theenvironments above the glass in the hot section and the cold section ofthe chamber are as discussed above.

[0042] The glass melt coming into the tin bath can contain water. Theglass melt can have a water content equal to or greater than 0.045weight percent based on the total weight percent of the composition.

[0043] The next step of the invention involves removing the float glassfrom the bath as is well known in the art.

[0044] After the float glass is removed from the float chamber, theglass is controllably cooled and cut into glass sheets. The sheet can befurther processed, e.g. cut to shape and heat processed, to form adesired glass article.

[0045] The glass can also be coated. In a non-limiting embodiment of theinvention, the glass is coated. The coating can include one or morecoating layers and/or coating films. The coating can be of any desiredtype. For example, but not to be considered as limiting, the coating canbe an electroconductive coating, a heatable coating, an antenna coating,or a solar control coating, such as a low emissivity coating.Non-limiting examples of solar control and antenna coatings aredisclosed in U.S. Pat. Nos. 4,898,789; 5,821,001; 4,716,086; 4,610,771;4,902,580; 4,716,086; 4,806,220; 4,898,790; 4,834,857; 4,948,677;5,059,295; and 5,028,579, which patents are herein incorporated byreference. Non-limiting examples of electroconductive coatings aredisclosed in U.S. Pat. Nos. 5,653,903 and 5,028,759, which are hereinincorporated by reference.

[0046] Glass made by a float process typically ranges from a sheetthickness of 2 millimeter to 20 millimeters. Glass having theaforementioned thickness can be prepared on a conventional float linehaving a line speed ranging from 100 to 800 inches per minute. Therequired thickness of the glass is determined by the end use of theglass.

[0047] The present invention provides glass having reduced defectdensity; specifically open-bottom bubbles. Such defects in glass can bemeasured using on-line and off-line methods. An Automatic InspectionSystem manufactured by Inspection Technologies Inc. can be used tomeasure defects on-line. Defects can be measured off-line by visualinspection. The defect density of glass is measured as number of defectsper 100 square feet. The standards for measuring defects in glass arewell known in the art. For example, defects can be measured incategories from <0.06″ to >0.25″.

[0048] Glass produced according to the present invention can meet thevarious commercial standards for defect density. For example, carmanufactures set standards for defect density for automotivewindshields. One automobile manufacture requires-automotive windshieldglass production to have less than 1 total defect per 100 square feet.

[0049] The glass produced according to the present invention can be usedas automotive transparencies, in colored glasses, laminated products,etc. as is well known in the art. A laminated product can comprise atleast one piece of glass produced according to the present invention.Such a laminated product can be a windshield.

EXAMPLES

[0050] The invention is illustrated by the following non-limitingexamples. The following is an example of a control run where hydrogenwas in the lower plenum of the hot end and a run according to thepresent invention. Control Example of the Invention H₂ in total chamber1900 scfh 600 scfh H₂ in at least lower 1300 scfh 0 scfh plenum of hotend Open Bottom Bubble 1.36 per 100 sq. ft. 0.07 per 100 sq. ft. defectsThickness of glass 3 mm 3 mm Tonnage 599 Tons per day 604 TPD ThresholdTemperature 1769° F. 1761° F. H₂O in glass 0.049% 0.049%

CONCLUSION

[0051] The apparatus and method of the present invention allows floatglass to be produced which has substantially reduced open-bottom bubbledefects as compared to conventional float glass.

[0052] The above examples are offered only to illustrate the presentinvention. The scope of the present invention is defined by thefollowing claims.

What is claimed is:
 1. A float glass chamber comprising: a hot sectionhaving an atmosphere in at least the lower plenum comprises less than 3percent hydrogen based on volume; and a cold section, wherein theboundary line between the hot section and the cold section is where thetemperature of the glass falls below a threshold temperature.
 2. A floatchamber according to claim 1 wherein the threshold temperature of thechamber is 1600° F.
 3. A float chamber according to claim 1 wherein thethreshold temperature of the float chamber is 1800° F.
 4. A floatchamber according to claim 1 wherein the threshold temperature of thefloat chamber is 2100° F.
 5. A float chamber according to claim 1wherein the atmosphere in at least the lower plenum of the cold sectioncomprises up to 10 percent of hydrogen based on volume.
 6. A floatchamber comprising: a hot section having an atmosphere in at least thelower plenum comprises less than 3 percent hydrogen based on volume; anda cold section, wherein the boundary line between the hot section andthe cold section is where the temperature of the glass falls below athreshold temperature of greater than 1600° F.
 7. A float chamberaccording to claim 6 wherein the atmosphere in at least the lower plenumof the cold section comprises up to 10 percent of hydrogen based onvolume.
 8. A method for making float glass with reduced defect densitycomprising: a. melting a glass composition to form a glass melt; and b.pouring the glass melt in a float chamber having a hot section and ancold section, the boundary line between the hot section and the coldsection is where the temperature of the glass falls below a thresholdtemperature, wherein the hot section has an atmosphere in at least thelower plenum comprises less than 3 percent hydrogen based on volume
 9. Amethod according to claim 8 wherein the threshold temperature of thefloat chamber is 1600° F.
 10. A method according to claim 8 wherein thethreshold temperature of the float chamber is 1800° F.
 11. A methodaccording to claim 8 wherein the threshold temperature of the floatchamber is 2100° F.
 12. A method according to claim 8 wherein theatmosphere in at least the lower plenum of the cold section comprises upto 10 percent of hydrogen based on volume
 13. A method according toclaim 8 wherein the glass melt has a water content equal to or greaterthan 0.035 weight percent based on the total weight percent of thecomposition.
 14. A method according to claim 8 wherein the float glassproduced comprises at least one piece of glass in a laminated product.15. A method according to claim 14 wherein the laminated product is awindshield.
 16. A method for making float glass with reduced defectdensity comprising: a. melting a glass composition to form a glass melt;and b. pouring the glass melt into a float chamber having a hot sectionand an cold section, the boundary line between the hot section and thecold section is where the temperature of the glass falls below athreshold temperature; c. pumping a gas mixture comprising less than 3%hydrogen based on volume into at least the lower plenum of the hotsection.
 17. A method according to claim 16 wherein the pumpingcomprises pumping a gas mixture comprising less than 1% hydrogen basedon volume into at least the lower plenum of the hot section.
 18. Amethod according to claim 16 wherein the glass composition comprises:from 65 to 75 weight percent SiO₂; from 10 to 20 weight percent Na₂O;from 5 to 15 weight percent CaO; from 0 to 5 weight percent MgO; from 0to 5 weight percent Al₂O₃; from 0 to 5 weight percent K2O; and from 0 to2 weight percent Fe2O₃, with weight percents being based on the totalweight of the glass composition.
 19. A method according to claim 16wherein the melting occurs in an oxy-fuel furnace.